The present invention relates generally to an imaging optical system well capable of taking images of objects in a wide-angle arrangement, and more particularly to an imaging optical system that is used on portable apparatus having an imaging optical system of small size, for instance, cellular phones and personal digital assistants.
With the recent widespread use of cellular phones, there are increasing demands for camera modules mounted on personal digital assistants. In turn, technologies capable of achieving further reductions in the size of an imaging device and enabling the imaging device to have much more pixels are under development. For an associated optical system too, a camera module having improved performance and thinned as much as possible is in great need.
So far, a telephoto type of ++− power construction has been known for the type that enables lenses to be thinned; however, as there are further size reductions, it is required to increase the positive powers of the first and second lenses as well as the negative power of the third lens. However, increasing the positive powers of the first and second lenses makes longitudinal chromatic aberrations likely to occur, and increasing the negative power of the third lens renders distortion worse.
Situations being like this, it is still difficult to make sure performance with a three-lens arrangement; in recent years, an optical system designed to achieve high performance with a four-lens arrangement has been proposed in the art too.
In the imaging optical system set forth in JP(A) 2004-184987, the first group having high axial marginal rays is made up of a cemented lens to make improvements in longitudinal chromatic aberrations. However, this type works against correction of distortion because of having an asymmetrical power profile with respect to a stop. Distortion is corrected by the off-axis positive refracting power of the image-side surface of the fourth lens; however, light rays are largely bent off the optical axis, resulting in occurrence of field curvature and astigmatism.
In the imaging optical system disclosed in JP(A) 2006-309043 too, on the other hand, the first group having high axial marginal rays is made up of a cemented lens to introduce improvements in longitudinal chromatic aberrations. However, the fourth lens is configured in such a way as to have positive refracting power with a distance from the optical axis thereby easing distortion; however, light rays are largely bent, ending up with large astigmatism.